1. Field of the Invention
The present invention relates to a rare earth metal-based permanent magnet having a thin and dense metal oxide film which exhibits an excellent corrosion resistance and which is formed on a surface thereof with an excellent adhesion, and a process for producing the same at a low cost and with little effect on the environment.
2. Description of the Related Art
A rare earth metal-based permanent magnet such as an R--Fe--B based permanent magnet represented by a Nd--Fe--B based permanent magnet and an R--Fe--N based permanent magnet represented by a Sm--Fe--N based permanent magnet is made of a material rich in resource and inexpensive, and has a high magnetic characteristic, as compared with a Sm--Co based permanent magnet. Therefore, particularly the R--Fe--B based permanent magnet is used at present in a variety of fields.
However, the rare earth metal-based permanent magnet contains R which is liable to be oxidized and corroded in the atmosphere. Therefore, when the rare earth metal-based permanent magnet is used without being subjected to any surface treatment, the corrosion advances from the surface due to the presence of a small amount of an acid, alkali or water to generate a rust, thereby bringing about the deterioration and dispersion of the magnetic characteristic. Further, if the rusted magnet is incorporated in a device such as a magnetic circuit, there is a possibility that the rust is scattered to pollute the surrounding parts or components.
There are various proposed processes for forming a metal oxide film as a corrosion resistant film on the surface of a magnet with the foregoing in view.
For example, Japanese Patent Application Laid-open No. 63-192216 describes a process for forming various metal oxide film by applying an Al compound, an Si compound or a Ti compound to the surface of a magnet and drying such compound.
Japanese Patent Publication No. 2-37081 describes a process for forming an Si oxide film by applying an Si-alcoholate to the surface of magnet and then heating and decomposing the Si-alcoholate.
Japanese Patent Applicaion Laid-open No. 63-301506 describes a process comprising applying a colloidal solution containing fine inorganic particles (SiO.sub.2) and then heating and solidifying the solution.
Japanese Patent Applicaion Laid-open No. 63-168009 describes a process comprising depositing a Ti compound onto the surface of a magnet and then subjecting it to a heat treatment at 700.degree. C. in an inert atmosphere.
In recent years, a reduction in size and down-sizing of parts have been advanced in electronic and appliance industries in which a rare earth metal-based permanent magnet is used. In correspondence to this, it is required that the magnet itself is reduced in size and cost. To meet this requirement, the surface treatment of the magnet must be carried out with higher dimensional accuracy (a reduction in thickness of a film and an excellent corrosion resistance in a thin film) , with an increase in effective volume and at a low cost. Further, consideration of the environment is indispensable at present, and the influences of a treating solution and the film itself on the environment should be taken into consideration.
In the conventionally proposed process, however, it is difficult to keep up with the recent situation.
More specifically, in the processes described in Japanese Patent Application Laid-open No. 63-192216 and Japanese Patent Publication No. 2-37081, a chemically unstable metal compound itself or a solution of such a chemically unstable metal compound merely diluted in a solvent is used. For this reason, the metal compound may be evaporated in the course of formation of a film, thereby producing pinholes in a formed metal oxide film. Such processes also suffer from a problem that metal compounds merely react with each other to form a film and hence, the adhesion between the film and the surface of a magnet is poor, and cracks are liable to be produced during cooling subsequent to the heating due to a difference in thermal expansion coefficient between the film and the surface of the magnet. Therefore, the corrosion resistance of the film is unstable and as a result, it is difficult to utilize such process for the formation of a thin film.
In the process described in Japanese Patent Application Laid-open No. 63-301506, the film formed of the fine inorganic particles is merely piled up on the surface of a magnet and hence, a dense film with no pinhole therein is not formed, and the adhesion of a formed film to the surface of a magnet is poor.
The process described in Japanese Patent Applicaion Laid-open No. 63-168009 has, in addition to the problems associated with the processes described in Japanese Patent Application Laid-open No. 63-192216 and Japanese Patent Publication No. 2-37081, problems that a heat treatment at a high temperature must be carried out and hence, there is a possibility that the deterioration of the magnetic characteristic of the magnet itself is brought about, and the producing equipment is large-sized, resulting in an increased cost, and that the process cannot be utilized for a bonded magnet which is not stable to heat.